Abstract: Monopole antenna is widely used as HF/VHF radio in vehicle system. Its response characteristics exposed to high-altitude nuclear electromagnetic pulse(HEMP) is simulated using CST in this paper. The time-domain and frequency-domain response voltages of antenna terminal's 50 Ω load are calculated. And its their variation with angle of incidence of electromagnetic pulse, top area of vehicle and antenna position in top surface are explored. The simulation results show that the maximum response voltage at feeding point increases as the increase of the angle between electric field vector of incident pulse and horizontal plane, and the increases of effective reflection area to incident pulse. The simulation results have important guiding significance for the antennas layout and the selection of electromagnetic pulse protection device for the antenna system.